In horology, the disassembly of a watch may be as difficult as the assembly thereof.
In particular, numerous sub-assemblies, such as movement-display assemblies, are fitted in the case from above, i.e. from the crystal side. This type of assembly generally includes, in addition to the movement, a dial and display members formed by hands or discs. The dial is in principle surmounted by a flange. The crystal is then driven into the middle part. It is usual to use a compressed air extractor to drive off the crystal, in order to access this assembly again.
CH Patent Application No 646028G in the name of MONTRES RADO SA provided a first response to this problem, by implementing a watch case which includes a middle part-bezel including an aperture that can be closed by a crystal, an upper housing and a lower aperture of smaller dimensions to those of the upper housing. This case further includes a crystal and a back cover removably secured to the middle part-bezel on either side thereof. The back cover is preferably fixed by screws to the middle part-bezel, and the crystal is held by slide bars. A casing ring, which is radially supported on the middle part-bezel via two stepped surfaces having different radii, at the level of the upper housing and the lower aperture, retains the watch movement radially and axially. In the assembled position, the casing ring is axially supported, via a lower gasket on the back cover on the one hand, and via an upper gasket and/or a flange on the crystal on the other hand. The casing ring, carrying the movement on a shoulder, can thus be introduced into the middle part-bezel on the upper housing side, and inserted into the lower aperture. The dial covers both the movement and the casing ring. The crystal is then set in place after inserting the upper sealing gasket and positioning the annular flange. On the opposite side, the back cover is screwed on, in a first variant onto the middle part-bezel compressing the lower sealing gasket against the casing ring, or in a second variant onto the casing ring compressing the lower sealing gasket and pressing a shoulder of the casing ring onto a shoulder of the middle part-bezel. Tightening the screws in both cases locks the casing ring inside the case. To disassemble the watch, the back cover and stem have to be removed, and the crystal taken off by pressing the lower portion of the casing ring, the pressure being transmitted to the crystal via the flange. This design allows cases of small thickness to be constructed, and the dimensions of the back cover to be reduced. However, the crystal must be disassembled either by a compressed air extractor, or by moving the casing ring and thus the movement.
In a similar manner, CH Patent Application No 629644G in the name of SAUCO Saunier & Cie discloses a casing ring which, from the back cover, extends over the entire height of the case, and directly supports the crystal. The removal of the crystal, together with the removal of the movement, is accomplished by pushing the lower portion of the casing ring.
Although these designs ensure correct sealing of the case, and the secure disassembly of the crystal and of the movement, they have the drawback of moving the casing ring and thus the movement, during the crystal removal operation.